1. Field of the Invention
This invention relates to glazings, and in particular to laminated glazings having a high intrusion resistance.
2. Summary of Related Art
Glazings for automotive use comprise safety glass which may be laminated (widely used for windscreens) or toughened (widely used for sidelights and backlights). Both types of glazing provide some degree of impact resistance, with laminated glazings having certain advantages over toughened glass so that, although laminated glazings are more expensive to manufacture than toughened glass, it would be desirable for all automotive glazings to be laminated to give improved intrusion resistance and to improve occupant retention in collisions. However, while conventional laminated glass (using polyvinylbutyral interlayer) provides better intrusion resistance than toughened glass, it will not resist a sustained attack especially when (as in the case of opening side lights) it is not permanently secured around its periphery by the glazing system used.
Attempts have been made to improve the impact resistance of laminated glazings by incorporating polycarbonate as an impact resistant ply in the laminate. Unfortunately, a polycarbonate ply at least 3 mm thick is required because polycarbonate is prone to stress cracking at lower thicknesses. However, while a 3 mm polycarbonate layer provides sufficient impact resistane for most purposes in a laminated glazing, when a 3 mm polycarbonate is laminated between glass panes (to provide the durability required for most uses) the resultant glazing becomes relatively thick and heavy, making it unsuitable for general automotive use.
A new family of engineering thermoplastic polymers have become available in recent years, the rigid thermoplastic polyurethanes, (RTPU), which are finding use in load-bearing engineering applications. Unfortunately, such polymers have poor resistance to ultra violet radiation, presumably due to inclusion of aromatic units in the polymer chains (the flexible thermoplastic polyurethanes currently used in aircraft glazings are aliphatic in character, and do not suffer the same deterioration on exposure to ultra-violet radiation). Thus they would not normally be considered suitable for use in external glazings.
We have now found that a rigid thermoplastic polyurethane ply may be satisfactorily protected from ultra-violet radiation so that it becomes suitable for external use by incorporating it in a laminate with an ultra-violet absorbing and/or reflecting ply outside (i.e. between the rigid thermoplastic ply and the source of incident ultra-violet light, normally the sun) the rigid thermoplastic ply, providing a durable, impact resistant glazing, suitable for automotive use, without the disadvantages of thickness and weight referred to above.